This invention relates to picture transmission systems. It has particular applicability to television systems, both closed circuit and broadcast television, but it is also applicable to facsimile systems, monitoring systems, and data transmission systems.
Current television techniques are based on a sequential lexicographic scan in which a sequence of horizontal lines is swept out point by point, with some interlacing to minimise flicker. The standard frame rate in Australia is 25 frames per second interlaced, which means, given the line structure, that a band width of approximately 5 MHz is required for acceptable transmission. Some minor improvement in band width is possible with refinements in picture encoding techniques, but even with these refinements the band width required runs well into the MHz range. If the scanning rate is decreased so as to permit transmission within a narrower band width, picture quality is adversely affected and there is severe flicker.
We believe that the difficulty in decreasing band width is due to the response of the visual system to systematic and, particularly, to linear scan patterns. We have found that picture analysis and transmission within band widths below 1 MHz with elimination of flicker can be achieved by scanning the picture in a random scan, or a pseudo-random scan.
The fact that use of a pseudo random scan makes possible a narrow-band transmission providing pictures of good entertainment value and high usable information content has previously been recognized.
Similar benefits are obtained with a random or pseudo-random scan in facsimile systems and other picture transmission systems.
However no prior picture transmission system has been described which enables a true random scan to be used -- in all cases a pseudo random scan has to be employed. In all cases the pseudo random scan is within a frame, or fields forming a frame, and synchronising signals must be transmitted along with the luminance signal in order to provide a reference by which the pseudo random generator is periodically reset to a standard value from which it steps along in synchronism with the generator at the transmitter.